Most materials behave elastically at low levels of strain. For crystalline solids and amorphous glasses, elasticity occurs up to a strain limit rarely exceeding 1%. Elastic strain is related to the extent to that atoms are dislodged from their equilibrium positions. However, elasticity in polymers is very different, and polymeric materials can exhibit elastic behavior to several hundred percent strain. Polymeric elastomers are usually high molecular weight molecules, well above their glass transition temperature, TG, and they typically contain a network of chemical or physical crosslinks that act as permanent entanglements and restrict long range (irreversible) slippage of chains. When a polymer elastomer is stretched, a restoring force arises because molecular chains are distorted from their most probable and preferable configuration—this phenomenon is known as entropic elasticity. Several classes of polymers exhibit entropic elasticity, including natural and synthetic rubbers and polyurethanes.
Entropy-based elasticity must be differentiated from the so called “shape-memory effect” defined by the literature. A shape-memory material is one that returns to its original shape only after the application of an external stimulus (Irie, “Shape Memory Materials.” Chapter 9: “Shape Memory Polymers” Otsuka and Wayman eds. Campbridge University Press, 1998). For example, a thermo-responsive shape-memory material returns to its “remembered” shape only upon heating past a critical shape-memory temperature TSM. Above TSM such a material can be elastically deformed by subjecting it to external stresses, and then cooling it (while under stress) beneath TSM. In the cooled state, external stresses can be removed and the material retains its deformed shape. Upon subsequent heating above TSM, the material recovers its elastic strain energy and returns to its original shape. Metallic alloys and ceramics are well-known to exhibit this shape-memory effect. Shape-memory polymers (SMP's) are noted for their ability to recover extremely large strains—up to several hundred percent—that are imposed by mechanical loading. The large-strain recovery observed in SMP's is a manifestation of entropy elasticity.
SMP's offer tremendous advantages to the fields of biotechnology and medicine (Lindlein et al., “Shape Memory Polymers” Angew, Chem. Int. Ed. 41, p 2034 (2002)). By exploiting the large-strain recovery of SMP's, surgeons can implant bulky objects into the body through small incisions. Biodegradable SMP's enable the development of degradable sutures and vascular stents. Biological MicroElectroMechanic Systems (Bio-MEMS) can perform intricate gripping, releasing, or even stitching operations. SMP's can also be used in non-biological applications including rewritable storage media, intelligent packaging materials, shapeable tools, and deployable objects for space exploration. SMP's can also be used in the development of recyclable thermosets and materials processing.